Magnetic resonance imaging apparatuses irradiate a radio frequency magnetic field of a specific frequency to a subject placed in a static magnetic field to excite nuclear magnetization of hydrogen nuclei contained in the subject (nuclear magnetic resonance phenomenon) and detect nuclear magnetic resonance signals generated by the subject to make it possible to obtain physical and chemical information. In the magnetic resonance imaging (henceforth abbreviated as MRI) currently widely spreading, images reflecting density distribution of hydrogen nuclei mainly contained in water molecules of a subject are obtained.
In contrast to such MRI, the method called magnetic resonance spectroscopy (henceforth abbreviated as MRS) enables to obtain spectra in which nuclear magnetic resonance signals acquired from a desired region (voxel) are separated for every molecular type on the basis of differences in resonance frequencies occurring due to differences of chemical bonds in various molecules containing hydrogen nuclei (henceforth called chemical shifts).
Further, whereas spectra are obtained for one region in MRS, the method of simultaneously obtaining spectra for many regions (pixels) to image for every molecular type is called magnetic resonance spectroscopic imaging (henceforth abbreviated as MRSI), and by using this MRSI, it becomes possible to visually catch concentration distributions of every metabolite (molecule) type contained in the living body.
When a living body is used as a measurement object of the aforementioned MRS or MRSI, concentrations of metabolites are usually extremely low in many cases, and therefore if the MRS or MRSI measurement is performed without suppressing signals of water of high concentration, weak signals of metabolites are hidden in the foot of the huge signal peak generated by water, and it will become extremely difficult to separate and extract signals of metabolites. For this reason, a pretreatment for suppressing water signals is conventionally performed immediately before performing excitation and detection with an MRS or MRSI measurement sequence.
In the treatment for suppressing water signals, in order to excite nuclear magnetization included in only water molecules, a radio frequency magnetic field is irradiated first, wherein the transmission frequency of the radio frequency magnetic field is adjusted to the water peak position, and the excitation frequency band thereof is narrowed to a level around the water peak width. Then, in order to separate phases of the nuclear magnetization in a large number of water molecules in an excited state and to make the vectorial sum of nuclear magnetization zero, a dephasing magnetic field gradient is applied (pseudo saturation). While the pseudo saturation of water magnetization continues, excitation and detection are performed with an MRS or MRSI measurement sequence to measure weak signals of metabolites. Further, since the signals of metabolites are very weak, integration is often performed many times in the conventional MRS or MRSI measurement in order to improve the signal-to-noise ratio (S/N). Moreover, in the MRSI measurement, in order to impart positional information, it is necessary to repeat the measurement according to the number of pixels to be measured with changing stepwise application intensity of the phase encoding magnetic field gradient.